Network Working Group O. Mazahir
Internet-Draft J. Padhye
Expires: May 18, 2013 R. Trace
Microsoft
S. Loreto
Ericsson
G. Montenegro
Microsoft
November 14, 2012
HTTP 2.0 Principles for Flow Controldraft-montenegro-httpbis-http2-fc-principles-00
Abstract
This document states the principles for flow control in HTTP 2.0.
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Mazahir, et al. Expires May 18, 2013 [Page 1]

Internet-Draft Principles for Flow Control in HTTP 2.0 November 20121. Introduction
HTTP/2.0 introduces multiplexed streams over a given TCP connection.
In HTTP 1.X, there is no interleaving of Request/Response pairs.
Thus, any flow control issues are mostly left to the underlying TCP
implementation. In HTTP 2.0, each Request/Response pair uses a
separate stream, sharing the same TCP connection with other such
pairs over different streams. All such streams will be vying for a
common underlying resource of a single TCP connection. Given that
this interaction among all the streams is not visible to the TCP
implementation, handling the interaction among them has to be solved
at the HTTP 2.0 multiplexing layer. There are issues of
prioritization, head-of-line blocking and flow control. Perhaps the
most complex aspect is that of flow control. It seems likely that
flow control will follow a path similar to what TCP's complex
dynamics have followed throughout the years. In particular, TCP
congestion control has seen a constant progress of improved
specifications based on measurements and research of the networking
community. What the TCP community recognized early on was that this
was a hard problem. Thus, the best course of action was to agree on
a minimal set of rules or principles (e.g., TCP "friendliness").
Many TCP algorithms are then possible as a (mostly) local
implementation issue giving rise to TCP Reno, Tahoe, Vegas, CTCP, and
many more.
Flow control for HTTP 2.0 multiplexing over TCP is also a complex
issue. This document proposes (1) a set of principles aimed at
preventing egregious behavior, while allowing for future and ongoing
improvement of flow control algorithms, and (2) a simple flow control
algorithm that could be implemented in the absence of better schemes
(TBD). Other flow control algorithms with subsequent improvements
should be specified in separate documents without encumbering nor
delaying the base HTTP 2.0 specification. This is similar to how the
myriad TCP congestion algorithms published so far have been specified
separately from the base TCP documents.
The goal of this document is to propose additional text to the
HTTP/2.0 specification. The starting point for HTTP/2.0, the SPDY
[I-D.mbelshe-httpbis-spdy] protocol, does not have much language with
respect to flow control. Hence, the text below is offered as a new
section or sections within the HTTP/2.0 document.
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Internet-Draft Principles for Flow Control in HTTP 2.0 November 20122. Principles for Flow Control in HTTP 2.0 Multiplexing
Flow control for Multiplexing in HTTP 2.0 must follow these
principles:
1. Flow control is hop by hop (where "hop" means an HTTP 2.0 hop),
and not end-to-end.
2. Flow control is based on window update messages. It is
essentially a credit-based scheme.
3. Flow control is directional. The client and server independently
advertise their flow control preference. It MAY be declared by
the receiver and MUST be heeded by the sender.
4. Flow control can be OFF or ON. It is OFF if no flow control is
advertised by a receiver, or if it declares "infinite" credit to
the sender.
5. HTTP 2.0 should only standardize the format of the window update
message and its semantics. In particular, the algorithms used by
the receiver to decide when to send window update messages, and
how much to update the window by, are not mandated in the spec.
The draft should, however, provide some illustrative examples.
NOTE: Whether flow control operates on a per-stream basis, on a per-
session (per-TCP connection) basis or on both a per-stream and a per-
session basis is TBD.
The spec will not define the algorithms the sender will use to manage
priorities among streams and to minimize head of the line blocking.
This is included for completeness, but is essentially independent of
flow-control.
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Internet-Draft Principles for Flow Control in HTTP 2.0 November 20123. Acknowledgements
This document was produced using the xml2rfc tool [RFC2629].
Mazahir, et al. Expires May 18, 2013 [Page 5]